1. Field of the Invention
This invention relates to microfluidic and fluid handling devices and the modification of pore surface chemistry of porous polymer monoliths and thermoplastic polymers by photoinitiated grafting, surface modification and functionalization.
2. Description of the Related Art
The current rapid development of microfabricated analytical devices is fueled by the need of significant improvements in speed, sample throughput, cost, and handling of analyses. A variety of applications involving, for example, sensors, chemical synthesis or biological analysis have already been demonstrated using the microfluidic chip format. More complex micro total analysis systems (μTAS) or ‘Lab-on-a-Chip’ are expected to be implemented by combining a variety of functional building blocks within the chip. Current approaches to μTAS largely rely on the use of inorganic substrates such as glass, silica, and quartz in which the desired network of channels and other features are prepared using etching processes. The popularity of these materials stems from the ease of design and fabrication of prototypes as well as small series of microfluidic chips using the standard methods of microelectronics such as patterning and etching.
However, the cost of the multistep wet fabrication of these microfluidic chips is high and the use of thermoplastic polymer materials instead of hard inorganics would enable the use of inexpensive ‘dry’ techniques such as injection molding or hot embossing. Consequently, there is growing interest in the development of polymeric substrates for the fabrication of microfluidic chips.
The chemistry of the surface of polymer-based devices is determined by the thermoplastic material used for their fabrication. For example, most of the commodity polymers available for this application are hydrophobic. These materials include for example polycarbonates (PC), poly(methyl methacrylate) (PMMA), polydimethylsiloxane (PDMS), poly(butylene terephthalate), and polyolefins such as polyethylene, polypropylene (PP), poly(2-norbornene-co-ethylene) (“cyclic olefin copolymer”, COC), and hydrogenated polystyrene (PS-H). As a result of strong hydrophobic interactions, their surfaces can capture specific compounds from solution passing through the channels, changing their concentration in the solution, thus negating their precise quantitation. In addition, any molecules deposited on the wall of the channel also continuously change the character of the surface further affecting both adsorption of other molecules and the reliability of quantitative assays.
Despite the undeniable success of microfluidic chip technologies in a variety of applications, some problems persist. For example, almost all of today's reported microfluidic chips feature open channel architecture. Hence, the surface to volume ratio of these channels is rather small. This is a serious problem in applications such as chromatographic separations, heterogeneous catalysis, and solid phase extraction that rely on interactions with a solid surface. Since only the channel walls are used for the desired interaction, these microdevices can handle only minute amounts of compounds. Packing the channels with porous particles that significantly increase the available surface area and also enable the introduction of specific chemistries into the device can solve the issue of limited surface area in the macroscopic devices.
Previously, a novel format of porous materials—rigid macroporous monoliths polymerized in situ within the confines of a mold have been developed. See Svec, F.; Fréchet, J. M. J. Anal. Chem. 1992, 54, 820; Svec, F.; Fréchet, J. M. J. Science 1996, 273, 205 and U.S. Pat. Nos. 5,334,310; 5,453,185; 5,728,457; and 5,929,214, which are hereby incorporated by reference in their entirety, which describe the compositions of these monoliths in chromatographic columns and methods of making them. The porous structure of these monoliths is well controlled by varying the composition of the polymerization mixture and the polymerization temperature. The attachment of chains of functional polymers to the reactive sites at the surface of the pores affords multiple functionalities emanating from each individual surface site, thus dramatically increasing the density of surface groups. This has been demonstrated in U.S. Pat. Nos. 5,593,729 and 5,633,290, which are hereby incorporated by reference in their entirety, that the pores of monoliths can be selectively chemically modified.
Grafting is another way of tailoring surface chemistry. Several methods have been used to graft polymers onto thermoplastic polymer surfaces including such widely diverse methods as flame treatment, corona discharge treatment, plasma treatment, use of monomeric surfactants, acid treatment, free radical polymerization and high energy radiation. See, for example, Uyama, Y. et al., Adv. Polym. Sci. 1998, 137, 1.
Attachment of chains of polymer to the sites at the pore surface within a generic monolith provides multiple functionalities emanating from each individual surface site and dramatically increases the density of surface functionalities. Examples of grafting and functionalization of porous polymers and monoliths using free radical polymerization initiation can be found in the art. Viklund, C. et al. in Macromolecules 2000, 33, 2539, incorporate zwitterionic sulfobetaine groups into porous polymeric monoliths. Peters, et al. have previously shown in U.S. Pat. No. 5,929,214, that thermally responsive polymers may be grafted to the surface of pores within a polymer monolith by a two-step grafting procedure which entails (i) vinylization of the pores followed by (ii) in situ free radical polymerization of a selected vinyl monomer or mixture of selected monomers. The thermally responsive polymer changes flow properties through the pores in response to temperature differences.
Surface photografting with vinyl monomers has been used for functionalization of polymer fibers, films and sheets as for example described by Rånby B. et al., in Nucl. Instrum. Methods Phys. Res. Sect. B, 1991, 151, 301. However, although photografting has been used for modification of flat two dimensional surfaces, photografting of three dimensional highly crosslinked porous polymer monoliths functionalize or bind them to polymer surfaces has not been demonstrated since these materials were generally assumed to be opaque or diffractive.